1. Technical Field of the Invention
The present invention relates generally to a fuel injection control system such as a common rail system for automotive diesel engines which is designed to activate fuel injectors to spray jets of fuel into each of cylinders of the engine through a sequence of fuel injection events, and more particularly, to such a system designed to cancel or reschedule the execution of the fuel injection events so as to eliminate overlap in time between the fuel injection events.
2. Background Art
There are known fuel injection control systems such as common rail injection systems which are designed to open each of injectors installed in one of cylinders of a diesel engine several times in each engine operating cycle (i.e., a four-stroke cycle) including intake or induction, compression, combustion, and exhaust in order to improve the fuel economy and quality of exhaust emissions, and minimize mechanical noise or vibration of the engine.
Some of the above type of fuel injection control systems are designed to control each of the injectors to perform pilot, main, and post injections of fuel into the engine. The purpose of the pilot injection is to promote or activate combustion of fuel in the cylinders and reduce combustion noise and vibrations of the engine. The purpose of the main injection is to move the piston of the engine. The purpose of the post injection is to re-burn particulate matter (PM) resulting from the combustion of an air-fuel mixture in the engine.
FIG. 7 illustrates an example of a circuit structure of the above type of fuel injection control system.
The fuel injection control system is designed to energize coils L1 to L4 of four injectors to spray the fuel into first to fourth cylinders #1 to #4 of a diesel engine. The fuel injection control system includes a typical microcomputer 50 made up of a CPU 55, a ROM 56, and a RAM 57. The microcomputer 50 works to turn on and off a switching device TRo of a step-up transformer 51 cyclically at high speeds to develop the voltage higher than a battery voltage VB at one of ends of a coil Lo which is opposite the other leading to the battery voltage VB. Such high-voltage is retained in a capacitor C through a diode Do. When the microcomputer 50 turns on a switching device TR1 connected to a downstream side of the coil L1 and a switching device TRk, it will cause the high-voltage to be released from the capacitor C, so that a peak current flows to the coil L1 through a diode D1 and a common terminal To to open the injector for the first cylinder #1. Afterwards, the microcomputer 50 turns off the switching device TRk and switches a switching device TRh between the on- and off-state cyclically to produce a flow of a constant current through the coil L1 through the diode D2 and the common terminal To, thereby keeping the injector for the first cylinder #1 open. The microcomputer 50 works to control operations of the injectors for the second to fourth cylinders #2 to #4 in the same manner. A diode D3 serves as a freewheeling diode.
FIGS. 8(a) and 8(b) demonstrate operations of the injectors for two of the injectors for the first and third cylinders #1 and #3 as an example. NE pulses are pulse signals to be outputted from a crank sensor installed on a crankshaft of the diesel engine at regular angular intervals of rotation of the crankshaft. The injection timing at which each of the injectors starts to spray the fuel into the engine is determined based on the angular position of the crankshaft or the NE pulses. For instance, the time the fuel injection event Q1a, as illustrated in FIG. 8(a), is to be initiated is determined based on input of a specified one of the NE pulse and the time required by the crankshaft to rotate through a selected angle. Specifically, the fuel injection event Q1a is initiated after elapse of the time required for the crankshaft to rotate through the selected angle following the input of the specified one of the NE pulses. Subsequently, the fuel injection event Q1b is initiated a time T1a after completion of the fuel injection event Q1a. Similarly, the fuel injection event Q1c is initiated a time T1b after completion of the fuel injection event Q1b. The same is true for the third cylinder #3. Specifically, the time the fuel injection event Q3a is to be initiated is determined based on input of a specified one of the NE pulse. The fuel injection event Q3b is initiated a time T3a after completion of the fuel injection event Q3a. 
FIG. 8(b) illustrates for the case where the diesel engine starts to accelerate from a steady state in which the diesel engine runs at a constant speed.
In the steady state, the injection timing at which each of fuel injection events Q1d, Q1e, Q1f, and Q3c are launched is determined in the same manner as that illustrated in FIG. 8(a). When the diesel engine has started to accelerate at time T, the microcomputer 50 works to change the injection timing to be synchronous with rising of the NE pulses (see Q3d) in order to increase the accuracy in controlling the injection timing. This will cause, as illustrated in FIG. 8(b), the injection timing for the third cylinder #3 to be advanced, which may result in an overlap of, for example, the fuel injection event Q3d with the fuel injection event Q1f for the first cylinder #1.
When the injection timings for the first and third cylinders #1 and #3 overlap each other, the microcomputer 50 is required to energize the coils L1 and L3 simultaneously. This will, however, result in a decrease in amount of current flowing through each of the coils L1 and L3 as compared with when only either of the coils L1 and L3 is energized. Particularly, when the peak current is supplied to the coils L1 and L3 simultaneously, it will drop in each of the coils L1 and L3, thus resulting in a delay in opening the injectors and a decrease in quantity of fuel injected into the engine. In order to avoid this problem, Japanese Patent First Publication No. 2005-299565 (US 2005/0229898 A1), assigned to the same assignee as that of this application, teaches a fuel injection control system engineered to extend the length of time the injectors are energized to achieve a desired quantity of fuel injected to the engine when the injection timings overlap each other.
The above type of fuel injection control system may, however, also encounters another drawback when the peak current is supplied to, for example, the coils L1 and L3 simultaneously. Specifically, the simultaneous supply of the peak current to the coils L1 and L3 may cause the current which is about two times that when the peak current is applied to either one of the coils L1 and L3 to flow through the switching device TRk, the diode D1, and the common terminal To, thus resulting in a large load thereon. For instance, when it is required to open the injector for the first cylinder #1, the microcomputer 50 works to apply the peak current I1 to the coil L1 through the common terminal To. When it is required to open the injector for the third cylinder #3, the microcomputer 50 works to apply the peak current I3 to the coil L3 through the common terminal To. When it is required to open the injectors for the first and third cylinders #1 and #3 simultaneously, the peak current I1 flows through the coil L1, while the peak current I3 flows through the coil L3, so that the sum of the peak current I1 and I3 flows through the switching device TRk, the diode D1, and the common terminal To.
The fuel injection control system, as taught in the above publication, eliminates the former problem, but has a difficulty in addressing the latter problem that devices other than the injectors are subjected to a large electrical load.
When it is determined that two injection durations will overlap each other, the one of the two injectors which is to be energized next may be inhibited from injecting the fuel into the engine to avoid the simultaneous flow of the peak current through the coils of the two injectors. This is, however, objectionable to running conditions of the vehicle. For instance, when drive torque is required to run the vehicle, but the execution of the main injection is cancelled, a difficulty may be encountered in ensuring the stability of running of the vehicle. Additionally, when the execution of the pilot injection is cancelled during acceleration of the vehicle, it may result in knocking of the engine or increase in combustion noise. Further, when a large amount of particulate matter (PM) is trapped in a diesel particulate filter (DPF), and the execution of the post injection is cancelled, it may result in a lack of burning the PM, thus decreasing the ability of emission gas purification.